In this competing renewal of AR036794 (title updated to reflect the evolution in specific aims and scope), we focus on a newly discovered mechanism of regulation of collagen fibril diversity that is important for understanding skeletal tissue differentiation and pathobiology. We have evidence that 3-hydroxproline (3Hyp) residues play a fundamental role in directing the manner of fibrillar collagen supramolecular assembly. The first hint came from our demonstration by mass spectrometry that the single, fully occupied 3Hyp site (P986) near the C-terminus of collagen a1(I) and a1(II) chains fails to be hydroxylated in collagens of the crtap mouse and recessive forms of human osteogenesis imperfecta (O.I.) caused by CRTAP or LEPRE1 mutations. We now have evidence for three classes of 3Hyp site in fibril-forming collagens. For example, a second site in type II collagen is highly hydroxylated in vitreous, meniscus and intervertebral disc but not in hyaline cartilage. The sequence motif of this second site is reproduced at D-periodic intervals in a2(V), with 3Hyp present, and shows similarities to a 3Hyp motif in type IV collagen. Tendon collagen uniquely contains a third type of 3Hyp motif, which we believe is characteristic and functionally important in tendon, ligament and related highly tensile tissues. Under 4 aims, we intend to pursue this concept aggressively since it is central to understanding how different cell types regulate the diversity of heteropolymeric collagen assemblies between different cartilages, bone, tendon and other connective tissues. If correct, it also has concept- changing implications for the field of vertebrate collagen biology. The clinical significance is in providing a molecular basis for understanding processes that cause cartilages and other collagenous tissues of low turnover to degenerate in the adult musculoskeleton in osteoarthritis, disc degeneration and related disorders of collagen framework failure. In addition, a full understanding of the effects of disrupting prolyl 3-hydroxylation in recessive forms of osteogenesis imperfecta we believe will reveal a molecular mechanism for brittle bone common to all forms of O.I. Such findings will also be significant for understanding qualitative changes in bone matrix that add significant risk of osteoporotic fracture in the population as a whole and a potential for novel biomarkers and therapeutic targets.